The high integration of a DRAM (dynamic random access memory) device leads to a decrease of the cell pitch size. It is therefore important to insure misalignment margin in a fabricating process of a Gigabit DRAM and less.
Owing to the limitation of photolithography and etching techniques, it is the most important and difficult to insure misalignment margin of a storage node contact to a gate electrode, a bit line, and a storage node during formation of a cell.
Referring to FIG. 1A, by forming a shallow trench isolation 12 on a semiconductor substrate 10, an active region and an inactive region are defined. A conductive layer (that is, gate electrode) is formed on the active region. The gate electrode has a structure that a silicide layer is layered on a polysilicon layer. A top surface and both sidewalls of the gate electrode are covered with a nitride layer spacer. A first oxide layer 14 as an insulating layer is formed to cover up the semiconductor substrate 10 including the trench isolation 12.
Then, a self-aligned contact (referring to below as SAC) process is performed. That is, using a contact hole formation mask, the first oxide layer 14 on the active region is etched to form a pad formation contact hole. After filling the contact hole with a polysilicon layer, the polysilicon layer is planarized to be coplanar with the first oxide layer 14 through a CMP (chemical mechanical polishing) process. As a result, a storage node contact pad 16 which is electrically connected to the semiconductor substrate 10 is formed. Simultaneously is formed a bit line contact pad (not shown) connecting a bit line to the semiconductor substrate 10 in a following process.
Referring to FIG. 1B, a second oxide layer 18 is formed on the first oxide layer 14 including the storage node contact pad 16. The second oxide layer is made of, for example, P-TEOS (plasma-tetraethylorthosilicate) with a thickness of about 1,000 .ANG.. Using a contact hole formation mask, the second oxide layer 18 is etched to form a bit line contact hole (not shown).
A bit line 20 which is electrically connected to the bit line contact pad via the bit line contact hole is formed. A third oxide layer 22, a nitride layer 24, and a fourth oxide layer 26 are sequentially formed on the second oxide layer 18 including the bit line 20. When a capacitor dielectric film is formed in the following process, O.sub.2 is created to oxidize the bit line 20. The nitride layer 24 may suppress the oxidation of the bit line 20. The total thickness of the second oxide layer 18, the third oxide layer 22, the nitride layer 24, and the fourth oxide layer 26 is about 6,000 .ANG..
Referring to FIG. 1C, the fourth oxide layer 26, the nitride layer 24, the third oxide layer 22, and the second oxide layer 18 are sequentially etched down to a top surface of the storage node contact pad 16 by using a contact hole formation mask. Thus, a storage node contact hole 27 is formed. If the mask is misaligned, it is possible to generate a short between the bit line and a storage node contact formed in the following process.
Referring to FIG. 1D, a storage node formation conductive layer is formed on the fourth oxide layer 26 including the foregoing storage node contact hole 27 as high as the height of a storage node. Using a storage node formation mask, the conductive layer is patterned to form a storage node 30 which is electrically connected to the storage node contact pad 16.
When the second oxide layer 18 is etched to form the bit line contact hole, the etching thickness of the oxide layer is relatively thin (about 1,000 .ANG.) and as soon as a contact pad is formed, the bit line is formed. Therefore, it is not difficult to align to the contact pad.
However, there can arise some problems associated with alignment between the contact pad and the storage node. For example, it is very difficult to form a buried contact (storage node contact 28) by etching a thick nitride layer (e.g., about 6,000 .ANG.). Furthermore, when aligning the storage node contact 28 to the underlying corresponding contact pad, various layers such as a gate electrode line, bit line contact, bit line and storage node serve as obstacles and they must be taken into account. As a result, it is very difficult to ensure enough mis-alignment margin (alignment tolerance) exceeding 50 nm.
However, generally mis-alignment margin between the gate electrode and the storage node contact is less than 40 nm and mis-alignment margin between a bit line and storage node contact is less than 40 nm. The 0.15 nm cell pitch requires storage node contact dimension of 100 nm at the top portion and 80 nm at the bottom portion thereof, but the critical dimension of the storage contact is increased due to the fact that layer to be etched is about exceeding 6,000 A and sufficient overetch process is needed. Therefore, the mis-alignment margin between the storage node and storage node contact becomes very narrow, for example, less than 20 nm.
Accordingly, there is a need for a method which can provide a sufficient mis-alignment margin between the storage node contact and the storage node without producing an electrical bridge.